1. Field
This invention pertains generally to a method of validating nuclear reactor in-vessel detectors and more particularly to such a method for validating the output signals of fixed incore flux detectors and core exit thermocouples.
2. Related Art
A pressurized water reactor has a large number of elongated fuel assemblies mounted within an upright reactor vessel. Pressurized coolant is circulated through the fuel assemblies to absorb heat generated by nuclear reactions in fissionable material contained in the fuel assemblies. An ex-core detector system mounted outside the reactor vessel provides a measure of the average power generated by the fuel assemblies. However, it is also important to note the distribution of power through the core to assure that operating limits are not exceeded. The power distribution is affected by a number of factors, such as for instance, the degree of insertion of control rods into the fuel assemblies.
Systems have been developed to determine the power distribution in a pressurized water reactor. One system known as the BEACON™ core monitoring system, available for licensing from the Westinghouse Electric Company LLC, Cranberry Township, Pa., employs a set of coupled, yet independent, computer software programs, which execute concurrently on one or more engineering work stations to generate on-line three-dimensional power distributions in the reactor core. The BEACON™ system uses an incore flux map together with a three-dimensional analysis to yield a continuously measured three-dimensional power distribution. The functions performed by the BEACON™ system include core monitoring and core analysis, including predictive functions such as online shutdown margin evaluations, estimated critical condition calculations and load maneuver simulation.
The flux maps in many nuclear plants are generated by running movable detectors through instrumentation thimbles in some, but not all of the fuel assemblies. In other plants, fixed incore detectors are positioned within the instrumentation thimbles and provide incrementally spaced axial flux readings at radially distributed locations throughout the core. The fixed incore detectors continuously provide a signal output that is used to map the core three-dimensional power distribution. The power produced in individual fuel assemblies can also be determined by the change in enthalpy of the coolant as it passes through the assembly. Enthalpy, in turn, is a function of the temperature rise over the fuel assembly, the pressure of the coolant and certain properties of the coolant. The coolant pressure remains fairly constant, but in any event, is a measured quantity, and the properties of the coolant are known. The rise in temperature is measured by inlet temperature sensors which measure the temperature of the coolant as it circulates back to the reactor core. Average inlet coolant temperature to the fuel assemblies can be measured accurately. Some, but not all, of the fuel assemblies are fitted with exit thermocouples. The enthalpy change in the instrumented assemblies can be calculated by measuring the temperature rise over the fuel assembly. If the coolant flow rate of the assembly is accurately known, then the power produced in the assembly is accurately obtained. A fuel assembly in a pressurized water reactor does not have an enclosure channel like boiling water reactors, which prevents the coolant from cross flowing among the neighboring assemblies.
The effect of the cross flow is characterized by the mixing factor which is defined as the ratio of the measured assembly power and the power determined from the measured enthalpy rise by the thermocouple. These mixing factors depend on the thermocouple location in the core and the reactor power level. These measured mixing factors are used to update the three-dimensional analytical nodal model of the power distribution. Power distribution uncertainties are evaluated by generating a standard deviation of the mixing factors from each thermocouple. These uncertainties are applied by the BEACON™ system to the measured power results.
Thus, the BEACON™ core monitoring system provides continuous monitoring of the reactor core three-dimensional measured power distribution and allows for an accurate assessment of available margin to various limits, e.g., peak linear heat rate, nuclear hot channel factor, and Departure from Nucleate Boiling Ratio (DNBR). To perform this monitoring function, the BEACON™ system relies on the accuracy and reliability of the self-powered neutron incore detectors and/or core exit thermocouples as a source of measurement information. There is no method currently within the BEACON™ system to automatically detect if one of these instruments is failing, failed, or providing an invalid signal. However, an invalid detector signal can cause inaccurate operating margins, which can lead to nonconformance of technical specification surveillance, unnecessary operation limitations on the plant, and can be time-consuming to diagnose the cause of the problem.
Accordingly, it is an object of this invention to provide a method that automatically goes through a series of evaluations on the data from each detector to determine if the detector output is valid.
It is a further object of this invention to automatically remove individual detector outputs from the core calculation considerations once the outputs have been verified as being invalid.
Additionally, it is an object of this invention to assure that detector outputs are not removed from consideration in the core calculations until it is verified that an acceptable number of remaining detector valid outputs are available to safely disregard the invalid detector outputs.